The invention relates to vertical measurement of light absorbance, comprising the absorbance measurement of a sample column of constant length. The invention is applicable especially when no reference sample is available or when a very exact measurement of the absorbance is required. The invention is applicable e.g. to determinations in clinical chemistry.
Under the Bouguer-Lambert Law, the absorbance of light passing through a sample is obtained with the formula
a=e*c*d 
in which
c is concentration
d is light path length and
e is constant.
In vertical measurement, the samples are usually located in wells that are open at their upper end. In that case, the height of the sample column is usually not known with sufficient accuracy. The curvature of the liquid surface also affects the determination of the exact height. This is no inconvenience if a reference sample with a known concentration is available. In fact, vertical measurement is then a preferable measuring method, because variations in the amount of diluting agent do not affect the measurement result.
In some applications, no reference sample is available. When a known substance is being measured, e is known. In order to obtain absolute concentration values, d must also be known with sufficient accuracy.
In horizontal measurement, the inner diameter of the cuvette determines the light path length d in the sample very accurately. This has been utilised also in some vertical photometers. They comprise a separate measurement duct, where absorbance is measured in a horizontal cuvette. This inherently results in a complex and expensive apparatus, since it requires measuring means both for horizontal measurement of ordinary wells, e.g., in a microtitration plate, and of a reference cuvette.
U.S. Pat. No. 5,963,318 discloses a vertical measuring cuvette having a lower measuring section and a neck part on the side of this. The cuvette is filled so that the liquid fills the measuring section to its total height, the free surface of the liquid being then on a higher level in the neck part. This allows vertical measurement at the measuring section through a liquid column with known height, e.g. a standard 10 mm liquid column. Such cuvettes can be measured in vertical photometry. However, the manufacture of such cuvettes is very troublesome; in practice they must actually be made by joining two pieces together. The manufacture of small cuvettes, e.g. having a diameter equalling the well in a microplate, is also troublesome. Thus, the cuvettes of the corresponding commercial application are relatively large-sized, and 8 such cuvettes can be placed in the rack of a 96-well microtitration plate.
A method of absorbance measurement, an insert used in the measuring well and an absorbance measuring apparatus according to the independent claims have now been found. A number of embodiments of the invention are described in the dependent claims.
In accordance with the invention, the measuring well comprises an insert that is pervious to light and supported by a stand, placed with its bottom underneath the sample surface. In this manner, the distance between the insert bottom and the measuring well bottom will remain constant in all cases, and the absorbance of a sample column with constant height can be determined. The sample surface may surround the insert bottom. The insert bottom may be a horizontal plane. The insert also has an upper surface. This may also be a horizontal plane. The invention also relates to a measuring well comprising an insert. The invention also relates to a method for measuring the absorbance of a sample with constant thickness.
In this method, the measuring light can be conducted from the top to the bottom or from the bottom to the top through the sample. Any disturbing air bubbles remaining between the insert bottom and the liquid sample may be removed e.g. by shaking before the measurement. The bottom may also be designed such that the bubbles are transferred beyond the area of measurement. The invention also relates to a measuring apparatus for determining absorbance through an insert.
The insert itself may be a well-like body. It may also be a closed body defined by the bottom and the upper surface.
The insert may be a body detached from the measuring well, and in that case it is placed into the well only when necessary. This allows the well to be used for conventional measurement if desired. In addition, the same insert can also be used in different wells, especially in wells with varied height. In connection with a detached insert, a special tool can be used to grip the insert. This tool is preferably such that grips the insert outside of the surfaces intended for conducting the measurement light. In this way the surfaces pervious to light in the measurement will remain as intact and clean as possible. The invention also relates to a tool for handling the insert.
The insert stand is supported by the well bottom, and thus the insert will be automatically positioned always at the same distance from the well bottom. The stand may also comprise centering means for centering the insert in the well. This allows light to be conducted from the same point both at the center of the well and of the insert in each case. The centering means may comprise one or more, e.g. three projections matching the well wall. The stand may have legs, which also act as centering means. In that case, they preferably act as springs and protrude by pressing against the well wall. The insert or the stand may also comprise centering means separate from the legs, e.g. lateral drags matching the well wall. This allows the legs to be supported by the bottom aloof from the corner between the wall and the bottom, because this angular shape may cause inaccuracies. This avoids the use of legs acting as springs, and consequently any distance errors caused by them. The separate centering means may have very high elasticity so that the same insert is usable in wells with markedly different widths.
At the upper end of the insert, a cover may be provided to close the well when the insert is in position in the well. The cover protects the sample and also prevents evaporation.
The measurements are usually performed on plates, each of which comprises several measuring wells. The most commonly used plate is the one called microtitration plate, which has 8*12 wells with a 9 mm distribution. If necessary, an inner well can be placed in one or more wells on the plate. The invention also relates to a plate having an insert in one or more of its wells. There may be individual inserts or several inserts combined to form strips or matrices, which are suitable for use on a plate with many wells.
The insert stand may have one or more legs. A separate insert may have such legs that press against the inner walls of the measuring well. In this manner, the insert will remain in position as well as possible. There may be e.g. 3 . . . 5, preferably 3 legs.
At the outer edge of the bottom of a stand supported on the bottom of the inner well, a deflecting means may be provided to avoid rounding of the well bottom angle. The deflecting means may be e.g. a rounding or a bevel. Owing to the deflecting means, the stand will always reach the bottom reliably.
The insert stand may be an integrated part of the insert. It may also be separately from the actual insert. In that case, it forms a base on which the actual insert is placed.
In accordance with the invention, the height of the absorbance is measured through a liquid column whose height is exactly known. The accuracy required in practical operation is of the order of 1%, which is easily achieved with the invention. Nevertheless, the measuring well can be made in the usual size. Any supplementary parts for normal use can be made by means of ordinary techniques of plastics commonly used in the field.